With regard to the operation of cellular phones used in mobile communication from the inside of a motor vehicle, to an increasing degree, antennas are being installed outside of the motor vehicle to improve the quality of the communication. In this way, depending on the installation location, the shielding effect provided by the vehicle's outer skin, which is generally made of electrically conductive materials, becomes negligible.
Since there are several cellular radio system providers which operate in different frequency bands, for example from 890 to 960 MHz in Europe, from 1710 to 1880 MHz for GSM, and, in the future, from 1920 to 2170 MHz for UMTS, there is a need for multiband antennas which cover these frequency ranges. Depending on the region, for example Europe and the Americas, these frequency bands are slightly offset from one another, so that an antenna that is optimized for Europe, for example, is not automatically suited for operation in the Americas.
Multiband, roof-mounted antennas, which are based on a monopole-type antenna structure, are often used. Monopole-type antennas have the advantage of an omnidirectional characteristic and of constant polarization ratios. With regard to roof-mounted antennas, current efforts are directed to achieving a lowest possible overall height. For reasons related to safety and to increased risk of injury in accidents involving pedestrians, bikers or motorcyclists, at present, heights of 4 cm are still required for roof-mounted antennas.
Moreover, there is a demand for additional measures to minimize the risk of injury in the event of an accident. As a result, antenna designs have become more complicated and expensive. Moreover, an antenna having a low overall height is able to be integrated more effectively in the general vehicle appearance, which is often a decisive criterion for the automobile manufacturers.
The height of an antenna is determined by the lowest frequency that it is designed to receive. In the case of a mobile radio antenna for Europe, this is 890 MHz. A height of approximately 8 cm (FIG. 1) is derived therefrom for a classic λ/4 monopole.
However, the overall height can be reduced by configuring the monopole in a meandering or folded shape (FIGS. 2 and 3). Such a reduction in overall height, however, is made at the expense of the attainable bandwidth. It is often precisely by using meandered or folded antennas, for example, that a communications frequency band for one region, such as Europe or the Americas, is able to be covered. A separate antenna must then be developed and provided for the other region in question.
Besides requiring low-height antennas, to an increasing degree, the automotive industry is stipulating that they be able to be used independently of the respective region. From the related art, disk monopole antennas are known, which are adapted to radiate from a lower limit frequency up to several GHz (FIG. 4 through 6). The overall height of such an antenna corresponds approximately to that of a λ/4 monopole for the same lower limit frequency. In the case of 800 MHz, this is approximately 8 cm.
Coupling elements have been proposed for generating additional resonances when working with a disk monopole. However, they produce only relatively narrow-band resonances, which are not capable, for example, of covering a complete lower 900 MHz cellular radio frequency band for Europe and the Americas.